To increase the capacity of transmission channels between terrestrial radio link stations or between earth stations and satellites, it is usual to use at the same time two carriers with equal frequencies and orthogonal polarizations, transmitted or received by the same reflector antenna with appropriate characteristics.
The carriers are generally separated by waveguide devices, so-called orthomode transducers, which are an integral part of the antenna feed; the transmission of respective signals to station apparatus is effected by means of separated waveguides or coaxial cables. The orthomode transducers must satisfy two requirements at the same time. They must ensure a satisfactory coupling of the radiofrequency signal between the antenna and transmission lines, consequently presenting a low stationary wave ratio. On the other hand, they must ensure a good isolation between the two access ports over a frequency band at least as wide as 10% of the mid-band frequency.
These electrical performances should be obtained by satisfying the mechanical requirements of maximum structural simplicity and small size. The latter property is important if the orthomode transducer is used in an antenna feed installed on board a satellite, either as an individual unit or as a part of an array. In the latter case, by reducing feed size, and hence feed weight and encumbrance, the satellite launching becomes simpler and cheaper.
In addition, to enable its use on board satellites, the transducer structure must have mechanical properties permitting it to remain efficient in spite of shocks suffered during the launching. More particularly, the number of parts which might change in position from a position ensuring the best electrical performance, such as the parts used for frequency tuning (namely screws), as a consequence of vibration should be minimized as far as possible.
An orthomode transducer is described on page 410 of the book entitled "Antennes micro-ondes" by Nhu BUI-HAI, issued by MASSON, in which the central conductors of two coaxial connectors are used as probes, oriented at 90.degree. with respect to one another and connected by a waveguide section. A metal plate is secured into this guide for the tuning of the parallel probe, as it acts as a short-circuit with respect to the radiofrequency signal.